With some further rumination, I'm afraid I'm going to disagree with Hyp's initial interpretation of the RAW. Reach weapons have a size of their own...but its implied rather than explicitly stated.
The PHB's rules on reach weapon state that reach weapons "double(s) the wielder's natural reach (PHB p113)."
It also says:
The measure of how much effore it takes to use a weapon...is altered by one step for each size cateogry of difference between the wielder's size and the size of the creature for which the weapon was designed...a small greatsword...is considered a one-handed weapon for a Medium creature, or a light weapon for a Large creature. Conversely, a Large Dagger...is considered a one-handed weapon for a Medium creature, or a two-handed weapon for a Small creature. If a weapon's designation would be changed to something other than light, one-handed, or two handed by this alteration, the creature can't wield the weapon at all. (PHB p113)
IOW, under those rules, a M creature cannot even wield a size L 2hd weapon at all. We all know this.
Like some other feats & racial abilities, Monkey Grip and Powerful Build are loopholes to a PHB rule, so were not considered by the original designers. They included several examples of creatures wielding inappropriately sized weapons in the text on p113 (which I edited out), but since no creature could wield an oversized reach weapon, they didn't consider that as a factor in wording the rules.
Both Monkey Grip and Powerful Build allow a creature to wield a weapon that is one size category larger than themselves, and to do so in a way that doesn't change the amount of effort required to wield the weapon. An oversized light, one-handed, or two handed weapon is wielded as if the wielder was of the same size class of the weapon.
Thus, a Large creature wields a 2 handed reach weapon and gains 10' of reach, defined
for him as double his natural reach (here, 10'). A size M creature normally couldn't use that weapon, but can because he has Monkey Grip or Powerful Build. He wields the weapon as if he were size Large, gaining 10' of reach...but since his
natural reach is 5' as opposed to 10', he can strike out 10'-15' feet as opposed to the 15'-20' of the Large creature. He still can't use it to attack creatures in
his natural reach (here, 5').
Of course, since its my PC and not my campaign, I'm not the final arbiter of how it will work in this case.
However, that's the way I'm going to run it IMC from now on.